This invention relates to an apparatus and method for drilling short radius curvature well bores which can be used particularly but not exclusively in the formation of a horizontal well bore at the bottom end of a conventional vertical bore.
Attention has recently been given to techniques for formation of horizontal bore holes at the bottom end of an existing vertical well. This technique is used in various situations in order to increase or recover lost production. It will be appreciated that one or more such horizontal bores can provide increased communication from the production area underground to the vertical bore hole.
In many cases such horizontal bore holes are formed using conventional drilling techniques which act to steer the drilling bit over a gradual curvature so as to move from the initial vertical orientation to a required horizontal orientation. However the conventional techniques require a radius of curvature of the order of several hundred feet so that there is a relatively large length of drilling involved and in addition it is necessary to carefully monitor the location of the drill bit to properly guide the bore holes to the required location.
The need therefore for a technique which will allow a short radius curvature to be developed is therefore apparent. Generally the intention is that the radius of curvature of the bore hole will be certainly less than one hundred feet and preferably of the order of or less than twenty feet. Such a very short radius of curvature allows the length of drilling necessary to reach the horizontal orientation to be relatively short. It is generally not necessary in such short radius curvature to utilize the highly expensive monitoring equipment necessary to properly guide and map the location of the drill bit since the distances involved are short and since it may be possible to utilize the geology of the site to act as a guide.
Attempts have been made to achieve such short radius drilling but to date have had little success due to problems in directional control, both in relation to azimuth and to rate of curvature.
In U.S. Pat. No. 4,858,705 (Thiery) is disclosed a technique for the formation of such small radius curvature bores, the radius of curvature in this patent being stated to be of the order of twenty to thirty meters. However this patent discloses a highly complex arrangement including an electric screw jack so that the equipment involved is extremely expensive and complex to operate.
Another technique known in the industry is to provide a flexible coupling between the drill bit and a down hole mud motor. The drill bit is then steered in its initial curvature by a curved guide surface known generally as a whipstock and the flexible coupling accomodates the curvature of the drilled bore. Examples of this arrangement are shown in U.S. Pat. No. 2,687,282 (Sanders); U.S. Pat. No. 4,495,595 (Holbert); and U.S. Pat. No. 4,456,080 (Holbert). This technique has the disadvantage that it is necessary to withdraw the whole drill string each time an extension piece of the flexible coupling is applied between the mud motor and the drill bit. In addition, this arrangement has had severe directional control problems with many attempts to solve these problems over the years which have not yet achieved success.
U.S. Pat. No. 3,878,903 (Cherrington) discloses a technique for drilling an arcately curved bore hole leading from an initial entry opening which is inclined at a shallow angle to the horizontal so that the bore hole then curves downwardly and then back upwardly to emerge at ground level beyond an obstacle such as a roadway or the like. This patent is therefore not concerned with nor useful with an initial vertical bore hole nor suggests how the device could in any way be used to develop a horizontal supplementary portion connected to an existing vertical well.